OFDM is an effective modulation technique that can mitigate the effect of multi-paths, and has been widely adopted in wideband communication systems, such as 802.11a/g, HIPERLAN/2 and recent 802.11n proposals (e.g., IEEE P802.11n TGn Sync Proposal Technical Specification, IEEE 802.11-04/0889r7, July 2005, and Enhanced Wireless Consortium (EWC) PHY Specification, v 1.0, September 2005). A disadvantage of OFDM is sensitivity to synchronization errors. In order to facilitate synchronization at the receiver side, training symbols, which are called “preambles,” are inserted at the beginning of each frame in most frame-oriented OFDM systems.
As described above, proposed schemes for frame timing synchronization can be divided into two categories: auto-correlation based synchronization techniques and cross-correlation based synchronization techniques. Auto-correlation based techniques exploit the repetition structure of preamble symbols or the guard intervals of data symbols to find the starting point of each frame. A well-known problem with conventional auto-correlation based techniques is a timing ambiguity caused by a plateau in the auto-correlator output signal, which will be described in more detail below.
Cross-correlation based techniques can be used to avoid the problems associated with conventional auto-correlation based techniques but, because the complexity of cross-correlation operations is much higher than that of auto-correlation operations, cross-correlation based techniques are limited to fine frame timing. Additionally, while conventional cross-correlation based techniques can work well in additive white Gaussian noise (AWGN) channels, these techniques may introduce some synchronization errors in multi-path channels, as will be described in more detail below.
What is needed, therefore, are robust techniques for frame timing synchronization that overcome the problems associated with conventional auto-correlation and cross-correlation based synchronization techniques.